1. Field of the Invention
This invention relates to an improved method of regulating polymer molecular weight in the preparation of homopolymers, copolymers and graft polymers of vinyl halides such as vinyl chloride, in which polymerization is carried out in the presence of an organotin mercaptide without the necessity of raising the polymerization temperature to obtain low molecular weight polymer.
2. Description of the Prior Art
The tendency of certain free radical polymerizable materials such as the vinyl halides to polymerize to relatively high molecular weight polymers under normal polymerization conditions is well known. These high molecular weight polymer products have relatively high melt viscosities so that they are generally processed with difficulty in processing procedures which require fusion of the polymer and handling of the molten polymer. Frequently the melt visocity of the polymer is so high as to require use of extremely high temperatures to obtain flowable or extrudable melts and such high processing temperatures can have a detrimental effect on the strength and color of the polymer.
Various techniques have been proposed for regulating the molecular weight of such polymers, i.e., for preparing polymers of lower molecular weight and, hence, lower melt viscosities. One such technique involves raising the temperature of the polymerization, but this procedure involves the danger of a "runaway" i.e., excessively violent, polymerization. Even when this danger can be avoided, e.g., by the inconvenient change of the free radical initiator for the polymerization, the high temperature molecular weight lowering technique provides polymers of undesirable long fusion time and/or high melt viscosities. Alternatively it is known to prepare the polymers of lower molecular weight by carrying out the polymerization in the presence of halogenated hydrocarbons such as perchloroethylene, chloroform, bromoform, ethylene dibromide and the like, or mercaptans such as alkyl mercaptans. However, many of these known molecular weight regulating agents require elevated polymerization temperatures which are nearly as great or as great as those of the elevated temperature regulating procedure described above. Additionally these agents are relatively volatile, importing unpleasant or even intolerably noxious odors to the polymer or the work area wherein the polymer is prepared or processed.
According to U.S. Pat. No. 3,272,786, to Perry, polymerization of an ethylenically unsaturated monomer, such as vinyl halide, in the presence of tin organo metallic compound wherein all of the organic substituents are attached to the metal by metal to carbon covalent bonds, for example tetraalkyl tin, e.g. tetraethyl tin, is effective in producing lower molecular weight polymer. However the reference compounds, which do not contain any sulfur, are of a structural type which is relatively volatile and in addition frequently toxic. Thus use of the reference molecular weight regulators in producing low molecular weight polyvinyl halide could readily produce toxicity problems in the work areas where the polymer is prepared and processed.